1. Field of the Invention
The present invention relates to an identification tag and more particularly to an identification tag having a large number of bits of information, where the identification tag has an electronic article surveillance function which is difficult to defeat.
2. Description of the Related Art
Radio frequency identification transponders (RFID Tags) have been developed in the last years to take advantage of the fall in semiconductor logic and memory prices. Such tags are available having a single silicon chip attached to a wire or patch antenna. Such tags, however, may be shielded from the high frequency RF used to communicate with the tags. The anti-theft properties of the RFID tags are suspect.
Magnetic electronic article surveillance (EAS) tags are much less easily shielded from the low frequency magnetic detection fields. Such EAS tags as described below, however, have possibilities of storing only a few bits of information.
Some conventional magnetic EAS tags have employed the Barkhausen jump effect. Generally, the Barkhausen effect is characterized by a tendency for magnetization induced in a magnetic material to change in discrete steps as an external magnetic field is increased or decreased. (The material is said to be a non linear magnetic material if the magnetisation of the material is not proportional to the external magnetic field.) A large temporal flux change, d.phi../dt, occurs when such a step takes place, and a sizable voltage may be induced in a sensing or pickup coil.
For example, U.S. Pat. No. 5,181,020 describes a thin-film magnetic tag having a magnetic thin film formed on a polymer substrate and a method for producing the same. The thin film exhibits a large Barkhausen discontinuity without intentional application of external torsional or tensile stress on use. A particular disclosed use is as a marker or tag for use in an article surveillance system wherein articles may be identified by interrogating the tagged article in a cyclic magnetic field of a predetermined frequency in a surveillance area and detecting a harmonic wave of the magnetic field generated by the tag in the surveillance area. This conventional system is only a single bit element using a single Barkhausen layer with no ability to develop a code to distinguish items.
U.S. Pat. No. 5,313,192 describes another single bit tag which relies on the Barkhausen effect. The tag of this invention is selected to include a first component comprised of a soft magnetic material which constitutes the bulk of the tag. A second component comprised of a semi-hard or hard magnetic material is integral with the first component. The tag is conditioned such that the second component has activating and deactivating states for placing the tag in active and deactivated states, respectively. Such conditioning includes subjecting the composite tag to predetermined magnetic fields during thermal processing stages. By switching the second component between its activating and deactivating states the tag can be switched between its active and deactived states. A reusable tag with desired step changes in flux which is capable of deactivation and reactivation is thereby realized.
U.S. Pat. No. 4,980,670 describes a one bit magnetic tag formed from a magnetic material having domains with a pinned wall configuration. The resulting hysteresis characteristic for that material is such that upon subjecting the material to an applied alternating magnetic field, the magnetic flux of the material undergoes a regenerative step change in flux (Barkhausen jump) at a threshold value when the field increases to the threshold value from substantially zero and undergoes a gradual change in flux when the field decreases from the threshold value to substantially zero. For increasing values of applied field below the threshold, there is substantially no change in the magnetic flux of the material. The tag may be deactivated by preventing the domain walls from returning to their pinned condition by, for example, application of a field of sufficiently high frequency and/or amplitude.
U.S. Pat. No. 4,940,966 describes the use of a plurality of magnetic elements in predetermined associations (e.g. with predetermined numbers of magnetic elements and with predetermined spacings between said elements), for identifying or locating preselected categories of articles. When the articles are caused to move relative to a predetermined interrogating magnetic field, each particular association of magnetic elements gives rise to a magnetic signature whereby the article or category of article carrying each of the predetermined associations can be recognized and/or located.
U.S. Pat. No. 4,660,025 describes a marker for use in an electronic surveillance system. The marker, which can be in the form of a wire or strip of magnetic amorphous metal, is characterized by having retained stress and a magnetic hysteresis loop with a large Barkhausen discontinuity. When the marker is exposed to an external magnetic field whose field strength, in the direction opposing the instantaneous magnetic polarization of the marker, exceeds a predetermined threshold value, a regenerative reversal of the magnetic polarization of the marker occurs and results in the generation of a harmonically rich pulse that is readily detected and easily distinguished.
U.S. Pat. No. 5,175,419 describes a method for interrogating an identification tag comprised of a plurality of magnetic, thin wires or thin bands which have highly rectangular hysteresis curves and different coercive forces. The wires or bands are preferably of amorphous material, but means for obtaining the highly rectangular hysteresis curves and different coercive forces are not taught; nor is the concept taught of using a time varying magnetic field superimposed on a ramp field for interrogation.
U.S. Pat. No. 5,729,201 describes an inexpensive multibit magnetic tag is described which uses an array of amorphous wires in conjunction with a magnetic bias field. The tag is interrogated by the use of a ramped field or an ac field or a combination of the two. The magnetic bias is supplied either by coating each wire with a hard magnetic material which is magnetized or by using magnetized hard magnetic wires or foil strips in proximity to the amorphous wires. Each wire switches at a different value of the external interrogation field due to the differences in the magnetic bias field acting on each wire.
The above identified U.S. patents are hereby incorporated by reference.
3. Related Applications and Issued Patents
Related U.S. patents assigned to the assignee of the present invention include: 5,528,222; 5,550,547; 5,552,778; 5,554,974; 5,538,803; 5,563,583; 5,565,847; 5,606,323; 5,521,601; 5,635,693; 5,673,037; 5,682,143; 5,680,106; 5,729,201; and 5,729,607. U.S. patent applications assigned to the assignee of the present invention include: Ser. No. 08/303,965 filed Sep. 9, 1994 entitled RF Group Select Protocol, by Cesar et al.; Ser. No. 08/621,784, filed on Mar. 25, 1996 entitled "Thin Radio Frequency Transponder with Lead Frame" by Brady et al. (pending); Ser. No. 08/626,820, Filed: Apr. 3, 1996, entitled "Method of Transporting RF Power to Energize Radio Frequency Transponders", by Heinrich et al.; application submitted Aug. 9, 1996 entitled RFID System with Broadcast Capability by Cesar et al.; application submitted Jul. 29, 1996 entitled RFID transponder with Electronic Circuitry Enabling and Disabling Capability, by Heinrich et al.; Ser. No. 08/592,250; Ser. No. 08/496,838; Ser. No. 08/496,838; Ser. No. 08/909,719; Ser. No. 08/621,784,660,249; Ser. No. 08/660,261; Ser. No. 08/790,640; Ser. No. 08/790,639; and Ser. No. 08/681,742. The above identified U.S. patents and U.S. patent applications are hereby incorporated by reference.